This invention is generally directed to improvements in color television projection systems. It is particularly directed to a method for correcting vertical non-linearity in a projected image, and a linearity correction network for use therewith.
A conventional CRT (cathode ray tube) used in color television receivers includes three electron guns. For development of a centered image, each gun directs its electron beam along an axis toward the geometric center of the CRT. If the guns fire off-axis, a circular image, such as the Indian head pattern, will not be tangent to the receiver's escutcheon at all points on the image, at least not without correction.
A common cause of off-axis firing is tipping of an electron gun. If one or more guns are tipped vertically off-axis, their beams are directed either above or below the center of the CRT's screen. This is shown in FIG. 1 which illustrates a CRT having an electron gun 10 which is tipped downwardly from a desired firing axis 12. If the tipping is not corrected for, the gun's beam travels along an axis 14 and strikes the CRT's screen below center.
To correct for such off-axis firing, a centering magnet 16 is conventionally employed. This magnet redirects the trajectory of the beam along a parabolic axis 18 so that the beam strikes the center of the screen.
Although centering magnets cause the beams of the electron guns to strike the center of the screen, the beams are not necesarily coincident at the top or bottom of the screen. Vertical compression of the image at the top or bottom of the screen and vertical expansion of the image at the other end of the screen typically result from the off-axis firing of one or more electron guns. Hence, vertical mis-convergence can result and must be compensated for.
In conventional direct view CRTs, all three electron guns tend to fire along the same axis. Thus, a correction which compensates for off-axis firing of one electron gun will normally be sufficient to compensate for the off-axis firing of the other two guns. This is not true, however, in projection systems which use three separate CRTs, each having its own electron gun.
To illustrate the typical mis-convergence which off-axis firing creates in a projection system, reference is made to FIG. 2. This figure shows the mis-convergence which results from a blue electron gun which fires off-axis in an upward direction by 0.060 inches, a green electron gun which fires off-axis in a downward direction by 0.120 inches, and a red electron gun which has no vertical firing error. The centering magnets are adjusted on the blue and green guns to render all images coincident at screen center.
The total mis-convergence was split differentially between the 6 o'clock and 12 o'clock position using a vertical height adjustment to achieve best convergence. The results shown indicate that the green raster ended up above the red raster by 0.060 inches at both the top and bottom of the screen. The blue raster ended up 0.045 inches below the red raster at the top and bottom of the screen.
To reduce the illustrated mis-convergence, conventional projection systems use, in addition to centering magnets, an additional deflection yoke, waveform shaping circuitry for driving the yoke, and a large number of service controls. Even with such complication and expense, however, the controls frequently require readjustment to maintain good convergence.